find() for std::set

c++, set, std, stl

Solution

The function that will be used in `std::set::find()` is an instance of the `Comparator` class that you declared as a template parameter of your set.

template < class Key, class Compare = less<Key>,
           class Allocator = allocator<Key> > class set;

Concretely, the comparator instance will be passed to Key objects and should return true if the first is located before the second. So yes, your implementation is fine.

Now, to dig further: if you want to convince yourself, you can dig into the source code of gcc’s implementation of the standard library, and you’ll find:

  template<typename _Key, typename _Val, typename _KeyOfValue,
           typename _Compare, typename _Alloc>
    typename _Rb_tree<_Key, _Val, _KeyOfValue,
              _Compare, _Alloc>::iterator
    _Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::
    find(const _Key& __k)
    {
      iterator __j = _M_lower_bound(_M_begin(), _M_end(), __k);
      return (__j == end()
          || _M_impl._M_key_compare(__k, _S_key(__j._M_node))) ? end() : __j;
    }

You can see that `_M_key_compare` (which is an instance of the `_Compare` class you provided), is called with `__k` as a first parameter (one of your `Key`) and the return value of `_S_key(...)` which is a key. The return value of `_M_key_compare` is used in a ternary expression, so it should be a boolean.

Problem

I have a question on how the objects stored in set are compared. ``` class A { public: char * name; }; ``` I am storing objects of A in a set. I am provide a comparator class which provides implementation of operator() ( A &ob1, A & ob2 ). Here I compare ob1.name & ob2.name and return true when ob1.name is lesser than ob2.name. Will I be able to search for an object using the find() of set ? I have provided implementation only for operator(). Will this be sufficient ? Could someone explain how find() works in this case ? Thanks in advance

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